Treatment: Participants are treated with escalating dosages (25/50/150/300 mg)of Lamotrigine/placebo once daily in two periods of eight weeks. Patients who prior to the study receive treatment, with drugs that can potentially influence myotonia, must stop the treatment during the study. Participants, who experience severe myotonia as defined by the MBS, are allowed to use escape medicine (Mexiletine) up until 60 hours before evaluation. Any use of Mexiletine will be noted and assessed as a part of the efficacy estimation. If Mexiletine is taken under 60 hours before evaluation blood concentration of Mexiletine is measured, to exclude data with blood concentration in the therapeutic level.